Solar cells based on CH3NH3MX3 and analogous metal (e.g., M=Pb or Sn) halide-based (hereinafter X or “halide”=F, Cl, Br, I or any combination thereof) materials with perovskite structure (referred to herein as “perovskites”) have demonstrated exceptional photovoltaic conversion efficiency and are among the most actively researched emerging photovoltaic technologies for future large-scale applications. Different deposition methods for perovskites have been reported, each with specific advantages and limitations.
For example, one deposition technique involves solution deposition from halide—CH3NH3I solutions. See, for example, S. Stranks et al., “Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber,” Science, Vol. 342 (October 2013) (hereinafter “Stranks”). While applicable for large-area printing applications, this approach described in Stranks does not readily produce continuous films. Further, most high-efficiency devices employing such a solution deposition approach rely on an additional nanoparticle scaffold of TiO2 or Al2O3 in order to minimize shunting effects. Such scaffolds typically require high sintering temperatures (e.g., exceeding 450 degrees Celsius (° C.)) for optimal performance which makes them inapplicable for tandem device structures on top of materials with low tolerance to temperatures above 200° C. (e.g., CIGS bottom cells).
Another deposition technique involves the co-evaporation of PbCl2 and CH3NH3I. See, for example, M. Liu et al., “Efficient planar heterojunction perovskite solar cells by vapour deposition,” Nature vol. 501, 395-398 (September 2013). While yielding high quality continuous films, co-evaporation with precise control of multiple fluxes is challenging and expensive to transfer to large-area manufacturing.
Yet another deposition technique involves the sequential solution deposition of lead halide and dipping in methylammonium iodide. See, for example, J. Burschka et al., “Sequential deposition as a route to high-performance perovskite-sensitized solar cells,” Nature, Vol. 499, 316 (July 2013) (hereinafter “Burschka”). Convenient, fast and scalable, this method however could only produce full conversion to the desired phase in devices employing additional nanoparticle scaffold of TiO2 (see Burschka) which, as provided above, requires high sintering temperature and thus makes the process inapplicable for device structures with a low tolerance to elevated temperatures. D. Liu et al., “Perovskite solar cells with a planar heterojunction structure prepared using room-temperature solution processing techniques,” Nature Photonics, 8, 133-138 (2014) (published December 2013) reports using ZnO as a support layer which can be processed at low temperatures since it does not require sintering. However, perovskite films were found to be highly reactive with ZnO films even at temperatures as low as 80° C. which could render these devices unsuited for outdoor applications.
Still yet another deposition technique involves sequential solution deposition and vapor anneal. See, for example, Q. Chen et al., “Planar Heterojunction Perovskite Solar Cells via Vapor-Assisted Solution Process,” J. Am. Chem. Soc. 2014, 136, 622-625 (hereinafter “Chen”) and associated supporting information (SI) (published December 2013). According to Chen, lead halide film samples were annealed for several hours at 150° C. on a hot plate, surrounded by CH3NH3I and covered by a Petrie dish at atmospheric pressure. See, for example, FIG. S1 in the associated supporting information of Chen. The temperature employed by this approach in Chen, especially for such a long duration may however be too high for many solar cell structures, including structures on Poly(3,4-ethylenedioxythiophene) (PEDOT) hole transporting materials and tandem structures with other bottom cells. Attempts to reproduce the approach described in Chen also revealed poor uniformity of the conversion over larger substrates (i.e., substrates larger than the 1 inch×1 inch used in research devices).
Therefore, there exists a need for an effective, low-cost and scalable method for large-area fabrication of perovskite absorbers, including those requiring lower processing temperatures in order to be compatible with the other solar cell elements.